Detailed Overview of Meiosis

Introduction

• Meiosis is the process by which germ cells in an organism undergo division to produce gametes for sexual reproduction.
• Unlike mitosis, which results in two diploid cells, meiosis results in four haploid cells.
• The process involves two rounds of division: Meiosis I and Meiosis II.

Basics of Mitosis vs. Meiosis

• Mitosis:
  • Involves one division cycle.
  • Results in two diploid cells.
  • Occurs in somatic cells.
  • Phases: Prophase, Metaphase, Anaphase, Telophase.
• Meiosis:
  • Involves two division cycles.
  • Results in four haploid cells.
  • Occurs in germ cells.

Meiosis I

• Interphase:

  • Cell prepares for division, DNA replication occurs.
  • Chromosomes exist in chromatin form, transition during S-phase to replicated chromatid pairs.

• Prophase I:

  • Chromatin condenses into visible chromosomes.
  • Homologous chromosomes pair up and undergo crossing over, exchanging genetic material at chiasmata.
  • Centrosomes replicate and migrate; spindle formation begins.

• Metaphase I:

  • Homologous pairs align at the cell equator.
  • Spindle fibers attach to kinetochores on centromeres.

• Anaphase I:

  • Homologous chromosomes separate to opposite poles.
  • Sister chromatids remain together.

• Telophase I and Cytokinesis:

  • Chromosomes arrive at poles.
  • Nuclear envelopes may form; cytokinesis divides the cell into two haploid cells.

Meiosis II

• Occurs in two haploid cells simultaneously.

• Prophase II:

  • Centrosomes replicate again.
  • Nuclear envelopes disassemble if they were reformed.
  • Spindle fibers reform and attach to chromosomes.

• Metaphase II:

  • Chromosomes line up at the equator again.
  • Each chromosome is composed of two sister chromatids.

• Anaphase II:

  • Sister chromatids are finally separated and pulled to opposite poles.

• Telophase II and Cytokinesis:

  • Nuclear envelopes reform around chromatids now considered chromosomes.
  • Cytokinesis splits each cell, resulting in four non-identical haploid gametes.

Key Differences and Importance

• Genetic Recombination:
  • Crossing over during Prophase I increases genetic diversity.
  • Random assortment during Metaphase I adds variation.
• End Product:
  • Four genetically unique haploid cells (gametes) from one diploid cell.
  • Essential for genetic diversity in sexual reproduction.